Tumor Motion Research Articles

Study Protocol to Quantify Breast-Chest Wall Motion in Free Breathing, Version 5.1

Background: Four-dimensional computed tomography (4D-CT) acquires multiple CT images over time, allowing to visualize the respiratory motion of organs and tumors. In patients undergoing 4D-CT for stereotactic radiotherapy of intra-thoracic tumors, 4D-CT provides the opportunity to study the incidental movements of the breast during normal breathing. A study protocol to investigate breast respiratory motion is presented herein. Methods: Patients with any primary cancer who underwent 4D-CT will be retrospectively selected from a radiotherapy treatment planning database. Study inclusion requires full lungs and breasts coverage in the 4D-CT images. Patients with advanced tumors are excluded. The protocol details breast locations of interest (upper, lower, inner, outer, central), specifies virtual breast tumor beds, and details how breast motion will be measured. Data will be analyzed to quantify the range of movements, to evaluate breast motion according to laterality and location, and to evaluate the correlation between movements of the breast and the intra-thoracic organs. Results: Results should provide quantitative information on the extent of breast movements during free breathing, which could be valuable to plan for respiratory control during breast radiotherapy. Conclusion: The study is retrospective, but the design of data acquisition is prospective, to mitigate recollection bias and enhance reproducibility.

Comparative evaluation of a surface-based respiratory monitoring system against a pressure sensor for 4DCT image reconstruction in phantoms.

Four-dimensional computed tomography (4DCT), which relies on breathing-induced motion, requires realistic surrogate information of breathing variations to reconstruct the tumor trajectory and motion variability of normal tissues accurately. Therefore, the SimRT surface-guided respiratory monitoring system has been installed on a Siemens CT scanner. This work evaluated the temporal and spatial accuracy of SimRT versus our commonly used pressure sensor, AZ-733V. A dynamic thorax phantom was used to reproduce regular and irregular breathing patterns acquired by SimRT and Anzai. Various parameters of the recorded breathing patterns, including mean absolute deviations (MAD), Pearson correlations (PC), and tagging precision, were investigated and compared to ground-truth. Furthermore, 4DCT reconstructions were analyzed to assess the volume discrepancy, shape deformation and tumor trajectory. Compared to the ground-truth, SimRT more precisely reproduced the breathing patterns with a MAD range of 0.37±0.27 and 0.92±1.02mm versus Anzai with 1.75±1.54 and 5.85±3.61mm for regular and irregular breathing patterns, respectively. Additionally, SimRT provided a more robust PC of 0.994±0.009 and 0.936±0.062 for all investigated breathing patterns. Further, the peak and valley recognition were found to be more accurate and stable using SimRT. The comparison of tumor trajectories revealed discrepancies up to 7.2 and 2.3mm for Anzai and SimRT, respectively. Moreover, volume discrepancies up to 1.71±1.62% and 1.24±2.02% were found for both Anzai and SimRT, respectively. SimRT was validated across various breathing patterns and showed a more precise and stable breathing tracking, (i) independent of the amplitude and period, (ii) and without placing any physical devices on the patient's body. These findings resulted in a more accurate temporal and spatial accuracy, thus leading to a more realistic 4DCT reconstruction and breathing-adapted treatment planning.

Long-Term Results of a Multi-Institutional Study of Dynamic Tumor Tracking-Stereotactic Body Radiotherapy for Lung Tumors

We had conducted a multi-institutional phase II study to evaluate the safety and efficacy of dynamic tumor tracking-stereotactic body radiotherapy (DTT-SBRT) for lung tumors. The primary endpoint was 2-year local control, which was reported to be 95.2%. After the initial evaluation of the primary endpoint, the study was extended as an observational study which was designed to evaluate 5-year survival and late toxicities. We present the long-term results of DTT-SBRT for lung tumors. The main eligibility criteria for the study were as follows: (1) primary or metastatic lung cancer with a diameter of 5 cm or less, and up to 3 lesions without any extrapulmonary lesions; (2) ineligibility to standard surgery, or patient's refusal of surgery; (3) ECOG-PS of 0 to 2; and (4) expected range of respiratory motion of 10 mm or more. The study included 48 patients from four institutions with the median age of 80 years (range, 49-90 years). Forty-two patients had primary non-small-cell lung cancer, and 6 patients had metastatic lung tumors. Forty-eight tumors (median diameter, 23.5 mm; range, 5-47 mm) in 48 patients were targeted for DTT-SBRT using a gimbal-mounted linear accelerator. Prior to treatment planning, spherical gold markers were placed around the tumor to detect internal tumor motion using fluoroscopy. The prescribed dose was 50 Gy in four fractions. Treatment beams were delivered with DTT according to a 4D model that predicts internal tumor motion with abdominal wall motion. DTT-SBRT was successfully delivered to all but one patient who had poor correlation between abdominal wall and tumor motion. Median follow-up period at data cutoff was 5.0 years (interquartile range, 3.1-6.3 years). Twenty-nine patients died; the causes of death were cancer-specific in 10 patients, comorbidity in 14 patients (pulmonary disease, renal failure, cerebral infarction, other malignancies, etc.), and unknown in 5 patients without cancer recurrence. Overall survival at 5 years was 51.5% (95% confidence interval [CI], 36.5-64.6%). Progression-free survival and local control at 5 years were 41.0% (95% CI, 27.0-54.5%) and 92.6% (95% CI, 78.7-97.6%), respectively. There were no grade 4-5 toxicities. One patient (2%) developed grade 3 radiation pneumonitis at 3 months. Grade 2 toxicities were observed in 9 patients (19%), including dyspnea, radiation pneumonitis, pleural effusion, rib fracture, and dermatitis. Dynamic tumor tracking SBRT achieved the long-term efficacy with low incidence of severe toxicities in lung tumors with respiratory motion. In this elderly patient cohort, non-cancer deaths were observed more than cancer-specific deaths.

Treatment Plan Creation and Delivery with and without BgRT for Static and Motion Trajectories

In this work we try to validate the motion tracking capabilities of BgRT for periodic and step motion trajectories. SBRT plans that matches the corresponding BgRT plans are created and delivered to the same phantom with and without motion and results are evaluated. Using BgRT based SBRT plans eliminates any user bias and creates SBRT plans that would represent treatment delivery scenarios that could have happened if the PET guided BgRT was not present for that treatment. To validate SBRT plans that matches the BgRT plans, we used three different types of motion patterns (1) static, (2) lung tumor motion and (3) one-centimeter step-shift. The lung tumor motion (∼25 mm in IEC-Y, ∼7 mm in IEC-X and ∼ 10 mm in IEC-Z) was used as it represents a continuous motion of the target for the entire length of the study while the step-shift case corresponds to the patient or tumor shifting between the localization CT and the start of treatment. First, a 10 Gy per fraction BgRT plan was created for each of the three experiments based on the corresponding PET image. Then, the BgRT plans were delivered to the corresponding targets with and without motion and results are evaluated. To perform a comparative study that assess the performance of BgRT and traditional SBRT (planning and delivery methods), the exact same plan fluence of BgRT plan for each experiment was used to create the corresponding SBRT plans. The newly created SBRT plans were delivered to the corresponding phantom experiments and were compared against BgRT delivery in terms of dose coverage and target margin loss using radiochromic film that moves with the target. The margin loss was calculated as the difference between the distance from the CTV contour to the 97% isodose contour in the treatment plan and the CTV contour to the 97% isodose contour on the film. Dosimetric coverage was on the other hand calculated as the percentage of the voxels within the CTV that lies within 97% and 130% of the prescribed dose. The results showed that the margin loss for BgRT is less than 3 mm, while for the SBRT plans were more than 3 mm when target motion is present. The dosimetric coverage for BgRT was 100% for all three cases, however less than 100% for the SBRT cases with motion. Table showing margin loss for the various experiments for a prescription dose of 10 Gy. The results shows that BgRT is capable of tracking the tumor motion and delivering the prescribed dose to the moving target.

The Feasibility of Fast, Four-Dimensional Computed Tomography-Based O-Ring Linac Plans for Stereotactic Body Radiotherapy in Patients with Poor Performance Status

As patients with poor performance status cannot endure long treatment times, it can lead to unplanned and prolonged radiation treatment or discontinuation of treatment. Moreover, prolonged radiation treatment appears to negatively affect survival for patients. We aimed to verify the feasibility of fast 4DCT-based O-ring linear accelerator (LINAC) treatment in lung 4D stereotactic body radiotherapy (SBRT). This study included data of 38 patients who received lung 4D-SBRT. Gating20-70% plans were based on 4DCT data obtained at phase values ranging from 20-70% using a C-arm LINAC. Non-GatingFB plans were retrospectively established based on 4DCT data obtained at phase values of 0-90% using an O-ring LINAC. The prescribed dose of 4,800 cGy was delivered to PTV in four fractions. The conformity index (CI), homogeneity index (HI), and gradient measurement (GM) of the PTV were analyzed to compare dosimetric data between Gating20-70% and Non-GatingFB plans. Organs at risks (OARs) were analyzed in accordance with the RTOG 0915 protocol. Treatment delivery time and total monitor units (MU) were analyzed to compare the efficiency of treatment delivery. Statistical comparisons were performed using the Wilcoxon signed-rank test (p<0.05). For the PTV, there was no significant difference in the CI or HI between Gating20-70% and Non-GatingFB plans (CI: 1.337±0.137, 1.335±0.174, HI: 0.939±0.015, 0.939±0.016, all at p>0.05). However, there was a significant difference in GM between Gating20-70% and Non-GatingFB plan (1.528±0.206 cm, 1.381±0.222 cm, p<0.001). For OARs, all plans met the criteria for dose constraint. There was a significant difference between Gating20-70% and Non-GatingFB plans except in the spinal cord. In particular, D1000cc and D1500cc values for the ipsilateral lung for Gating20-70% plans were 31.6% and 59.9% lower than those for Non-GatingFB plans (D1000cc: 50.780±132.061 cGy, 34.755±102.480 cGy, p<0.001, D1500cc: 6.641±14.598 cGy, 2.666±7.926 cGy, p<0.001). Treatment delivery time was 92% longer for Gating20-70% plans than for Non-GatingFB plans (5.0±0.4 min, 9.6±3.3 min, p = 0.043). The total MU value for Gating20-70% plans was 9.6% higher than that for Non-GatingFB plans (3665.8±230.7, 4016±236.9, p = 0.039). When compared to OARs both plans were acceptable per RTOG-0915 protocol, but significant differences were between Non-GatingFB and Gating20-70% plans. However, treatment delivery time of Non-GatingFB plans decreased by 47.9% than Gating20-70% plans. We verified the feasibility of fast 4DCT-based O-ring LINAC treatment for patients who cannot endure long treatment times due to poor performance status in lung 4D-SBRT. In further study, we will analyze dosimetric differences and treatment delivery efficiency considering tumor motion according to tumor location.

Assessment of Daily Target Motion to Optimize ITV Margins in Online Adaptive Plan for Liver and Pancreatic SBRT on MR-LINAC

Accurate measurement of daily target motion on online plan adaptation is essential for re-optimizing the internal target volume (ITV) margins. In this study, by analyzing 2D cine MR images acquired in a 1.5T MR-Linac, we assessed daily target motion and its inter-fractional variations, which is then used to customize ITV margins for MR-based online adaptive plan of liver and pancreatic stereotactic body radiotherapy (SBRT). Twenty-threepatients, enrolled in this study, underwent MR-guided online adaptive SBRT on 1.5 T MR-Linac: liver (n = 12 without compression belt (CB), n = 2 with CB) and pancreas (n = 9 with CB). Target motion in a 2D coronal and sagittal MR images (> 200 cine images to cover multiple breathing cycles), as well as a set of 4D-CT images during the simulation, was analyzed to determine patient specific ITV margins for a reference plan. On every fraction of online adaptive SBRT, the 2D MR images were exported to commercially available software along with a planning MR images: a T1-wighted MR image set for pancreas and or a navigated T2-wighted MR image set for liver. Daily target motion was then measured by overlaying the orthogonal cine image set on the T1- or T2-wighted image set. The ITV margin was adjusted when required prior to online plan optimization RESULTS: Thisstudy successfully assessed the daily target motion and inter-fractional variations in 23 liver or pancreatic cancer patients. We found a discrepancy of daily target motion of orthogonal 2D cine MR images from that of 4D-CT, required to adjust the ITV margin for online plan adaptation. In liver SBRT without CB, 15 out of 45 fractions from 12 patients (33.3 %) went through adjusted ITV margin. The tumor motion was large up to a 2.2 cm along the inferior direction in a patient without CB and thus the patient was treated on a conventional Linac with the active breathing control (ABC) unit for breath-hold. On the other hand, no adjustments were necessary for 2 liver patients with CB. In 9 patients with pancreatic cancers, we found the inter-fractional ITV variations in 3 out of 45 fractions (6.7 %) adjusted the ITV margin. The daily target motion in the pancreatic patients with CB was more stable and relatively smaller than that of the liver patients. Theorthogonal 2D cine MR images are evidently essential for assessing daily target motion to evaluate the inter-fractional variation and achieve daily ITV-customized liver and pancreatic SBRT in the 1.5 T MR-Linac. The customized ITV approach will improve the treatment outcome of the MR-guided online adaptive therapy.

4D-MRI Guided Stereotactic Body Radiation Therapy for Unresectable Colorectal Liver Metastases

This study evaluated the feasibilities and outcomes following four-dimensional magnetic resonance imaging (4D-MRI) guided stereotactic body radiation therapy (SBRT) for unresectable colorectal liver metastases (CRLM). From March 2018 to January 2022, we identified 76 unresectable CRLM patients with 123 lesions who received 4D-MRI guided SBRT in our institution. 4D-MRI simulation with or without abdominal compression was conducted for all patients. The prescription dose was 50-65 Gy in 5-12 fractions. The image quality of computed tomography (CT) and MRI were compared using the Clarity Score. Clinical outcomes and toxicity profiles were evaluated. The 4D-MRI significantly improved the image quality compared with CT images (mean Clarity Score: 1.67 vs 2.88, P < 0.001). The abdominal compression significantly reduced motions in cranial-caudal direction (P = 0.03) with 2 phase T2 weighted images assessing tumor motion. The median follow-up time was 12.5 months. For 98 lesions assessed for best response, the complete response, partial response and stable disease rate were 57.1 %, 30.6 % and 12.2 %, respectively. The local control (LC) rate at 2 year was 97.3%. 46.1% of patients experienced grade 1-2 toxicities and only 2.6% patients experienced grade 3 hematologic toxicities. The 4D-MRI technique allowed precise target delineation and motion tracking in unresectable CRLM patients. High LC rate and mild toxicities were achieved. This study provided evidence for using 4D-MRI guided SBRT as an alternative treatment in unresectable CRLM.

Toward Global Consensus for MR-Guided Treatment Planning for Pancreatic Tumors on a 1.5 T MR-Linac

MR-guided SBRT with a 1.5 T MR-Linac is a relatively new therapy for pancreatic tumors with varying expertise levels. Moreover, treatment planning in the upper abdomen can be challenging as target coverage is often compromised by dosimetric constraints of abutting bowel structures. This may lead to large differences between centers in protocols, practices. To increase harmonization a worldwide consortium was founded among 1.5 T MR-Linac users. In this work we report on the outcome of the first phase within this collaboration, which is the assessment of the baseline variation between the treatment planning protocols and subsequent dose distributions. Twelve centers across three continents (North America, Europe, and Australia) participated in this consortium. Each center was sent the same two anonymized data sets reflecting two cases of locally advanced pancreatic cancer of different complexity levels. The data sets included a CT scan, a predefined structure set containing the gross target volume (GTV) and the OARs, a brief medical history, tumor motion characteristics, and auxiliary CT and MR imaging. Centers were asked to create an MRgRT treatment plan according to their clinical five-fraction SBRT protocol, using their institutional margin structures, beam setup, target prescriptions, and OAR constraints. Key DVH parameters that were evaluated are D99%, D90%, D50%, D1% for the GTV and D0.5cc for the duodenum, small bowel, and stomach. In general, large variations were observed in planning objectives and machine settings yielding widely varying inhomogeneous dose distributions to both the tumor and organs at risk (Table 1). This was especially manifest for case 2 where the tumor abutted with both the duodenum and small bowel over a trajectory of multiple centimeters. Not only were different trade-offs between target coverage and OAR sparing observed, but also different strategies for optimizing the integral dose to the tumor. These results indicate a large variety in the treatment planning strategies that could well translate to differences in outcome. Based on this first evaluation, the consortium will work towards a collective consensus protocol with a second evaluation round after internal discussions.

Real-Time 3D Liver Tumor Localization via Combined Optical Surface Imaging and an X-Ray Projection from Arbitrary Imaging Angles

A deep learning (DL)-based, deformable registration-driven liver tumor localization technique was developed for onboard deformable motion tracking. The technique (Surf-X360-Bio) uses in-room optical surface imaging and an x-ray projection at an arbitrary scan angle to solve volumetric liver and liver tumor motion in real-time. Surf-X360-Bio solves the volumetric motion of the liver and localizes the liver tumor, through deforming liver and liver tumor meshes segmented on prior 4D-CTs/CBCTs. It uses real-time onboard information from an optical surface image and a simultaneously-acquired x-ray projection (from an arbitrary scan angle). Surf-X360-Bio localizes tumors via two steps: liver boundary motion estimation and intra-liver motion derivation. Surf-X360-Bio first estimates liver boundary motion by a patient-specific surface imaging model (Surf), utilizing the correlation between the external body surface motion and the internal liver boundary motion. As the correlation can be imperfect, the residual motion estimation errors were corrected by a patient-specific, angle-agnostic x-ray imaging model (X360). X360 deformed the liver boundary to match motion-related imaging features encoded in an arbitrarily-angled x-ray projection, using the Surf output for initialization. X360 adopted a geometry-aware learning module to extract and adapt to angle-varying features of arbitrarily-angled x-ray images, by calculating the projection system matrix of each x-ray image on-the-fly during model training and inference. After the liver boundary motion estimation by Surf and X360, intra-liver deformation was solved by a biomechanical model (Bio) to propagate the liver boundary motion inside to localize the tumors. The DL-based Bio model used domain knowledge of tissue biomechanics and finite element analysis (FEA) to solve intra-liver motion, with a much faster speed than conventional FEA methods to meet the real-time constraint. Surf-X360-Bio was evaluated using a dataset of 34 liver patients. Liver tumor localization accuracy was evaluated by the Dice similarity coefficient (DSC), the 95-percentile Hausdorff distance (HD95), and the center-of-mass error (COME). Using 3,306 motion scenarios spanning the 360 degree x-ray scan angles for each testing patient, Surf-X360-Bio localized the liver tumors to 0.81 (mean) ± 0.16 (s.d.) in DSC, 2.5 ± 1.7 mm in HD95, and 2.1 ± 1.8 mm in COME. In comparison, the prior reference image without deformable registration-driven localization yielded 0.42 ± 0.29 in DSC, 8.1 ± 5.2 mm in HD95, and 8.5 ± 5.2 mm in COME. Via Surf-X360-Bio, the overall inference time was below 230 ms for each case. Combining optical surface imaging and x-ray imaging, Surf-X360-Bio achieved fast (<250 ms inference time), accurate (mean error < 2.1 mm), and robust liver tumor localizations at arbitrary x-ray scan angles for real-time, marker-less 3D deformable motion tracking.

New Data-Driven Gated (DDG) PET/CT for Radiation Treatment Planning of NSCLC

PET/CT for RT simulation of NSCLC is typically conducted in two separate imaging sessions of gated 4D-CT in radiation oncology and fusion with non-gated PET/CT in diagnostic imaging, making treatment response assessment (TRA) with PET/CT and radiation treatment planning (RTP) challenging. To remedy this short-coming, we designed a new data-driven gated (DDG) PET/CT, based on the self-gated PET and self-gated CT data to derive the gated PET/CT data free of tumor blurring and misregistration artifacts without any respiratory monitoring device (ease of use and cost reduction). The new DDG PET/CT also provides the important 4D-CT information of end-inspiratory (EI), end-expiratory (EE), maximum-intensity-projection (MIP) and average-intensity-projection (AIP) CT's. The total imaging time is < 15 min, which makes DDG PET/CT simulation in a single session without any respiratory monitoring device possible. Acquisition is standard PET/CT imaging protocol followed by a cine CT imaging of the tumor area. We utilized the commercial DDG PET from GE Discovery PET/CT and designed a new DDG CT for attenuation correction of the DDG PET for reduction of tumor motion and mis-registration. The AIP and MIP images were from the average and maximum pixel values of the cine CT images, respectively. The cine CT images with the largest and smallest average CT number in the lung region were identified as the EE and EI CT images, respectively. For the image slices without any lung region present, the largest and smallest expansions of the body outline contour were selected to be the EE and EI CT phases, respectively. The AIP and EE CT images were used for attenuation correction of the PET and DDG PET data, respectively. Both DDG PET/CT and 4D-CT of (EI, EE, AIP and MIP) are for RT simulation of NSCLC. 38 4D-CT patient data sets were compared at the EE and EI phases between our DDG CT and 4D-CT to demonstrate the applicability of the newly designed DDG CT. A prototype software on a Dell PC of i5-6500 CPU has been successfully developed to enable DDG PET/CT and 4D-CT on seven GE Discovery PET/CT scanners. The operating system was Ubuntu and the computer language was Python. In the EE phase, the images selected by DDG CT and 4D CT were identical 62.5±21.6% of the time. In the EI phase, the images selected by DDG CT and 4D CT were identical 68.2±18.9% of the time. Inspection of the EE and EI phases of DDG CT and 50% and 0% of 4D-CT demonstrated both data sets of DDG CT and 4D CT were almost identical. A new DDG PET/CT and 4D-CT in a single imaging session without any hardware gating has been developed for TRA and RTP of NSCLC. The utility is available on the network and it enables all PET/CT scanners on the network for TRA and RTP. The new DDG CT provides the benefits of 4D-CT without any requirement of external hardware gating and make possible DDG PET/CT free of tumor motion and misregistration artifacts.

Heatmaps to Assess Tumor Motion Probability with MRcine in Head and Neck Cancer

Intrafraction motion affects tumor position during radiation for head and neck cancers (HNC) and can be assessed using magnetic resonance cine (MR-cine). Heatmaps - visual representation of patient specific temporal tumor location akin to internal target volume (ITV) - were analyzed to demonstrate the variation of population-based motion margins in primary oropharyngeal (OP) and laryngeal/hypopharyngeal (LH) sites with the goal to provide insights towards personalized margins. MR-cine were performed for LH and OP HNC patients as part of simulation protocol. Images (900-1500 slices) were acquired across 3-5 minutes per patient. Gross target volumes were propagated on T1 contrast sagittal sequence using deformable image registration then adjusted manually. Tumor locations were integrated across all frames and displayed as heatmaps representing location probability. To determine individualized motion, a baseline contour representing average tumor rest position was expanded both isotropically and directionally in 1 mm increments in a novel analysis to define the contribution of each increment on target coverage. To assess directional dependence, displacements from rest position were evaluated in 4 planes from centroid: (A) 12 o'clock (OC) to 6OC (capturing volumetric shift in a portion of superior, posterior and inferior directions), B) 3OC-9OC, C) 6OC-12OC, D) 9OC-3OC. Histograms demonstrating the proportion of scan time the tumor was within expansions were generated, with expansion margins for 25%-95% coverage analyzed by patient. Wilcoxon-rank-sum test was performed to compare motion by site. Motion was evaluated in 66 patients (LH n = 27, OP n = 39). In LH, a median (med) isometric expansion of 8.5mm to achieve 95% target was required, with large variations observed for minimum (min): 2.8mm and maximum (max):23.5mm. Smaller differences were observed for OP with med of 5.3mm, (min:1.9mm, max:14.6mm). At 75%, 50% and 25% coverage, expansions for LH (med: 4.8 min: 1.9 max: 17.3; med: 3.8 min: 1.2 max: 12.9; med: 3.3, min: 0.6, max: 9.3) were not significantly different than OP (med: 3.8, min: 1.7, max: 10.9; med: 3.1 min: 1.3, max: 9.4; med: 2.1, min: 0.7, max: 8.4), suggesting larger but infrequent shifts in LH. Directional tumor displacement varied widely (Table 1), and by site and points describing the need for personalized margins. Current literature as well as the analysis in this study indicate a wide range of motion - both in magnitude and duty cycle - between and within HN sub-sites not captured on CT simulation. Moving forward, motion based heatmaps based on MR-cine may serve as visualization tool for ITV contouring or to generate personalized motion margins.

Exploring beamline momentum acceptance for tracking respiratory variability in lung cancer proton therapy: a simulation study

Objective. Investigating the aspects of proton beam delivery to track organ motion with pencil beam scanning therapy. Considering current systems as a reference, specify requirements for next-generation units aiming at real-time image-guided treatments. Approach. Proton treatments for six non-small cell lung cancer (NSCLC) patients were simulated using repeated 4DCTs to model respiratory motion variability. Energy corrections required for this treatment site were evaluated for different approaches to tumour tracking, focusing on the potential for energy adjustment within beamline momentum acceptance (dp/p). A respiration-synchronised tracking, taking into account realistic machine delivery limits, was compared to ideal tracking scenarios, in which unconstrained energy corrections are possible. Rescanning and the use of multiple fields to mitigate residual interplay effects and dose degradation have also been investigated. Main results. Energy correction requirements increased with motion amplitudes, for all patients and tracking scenarios. Higher dose degradation was found for larger motion amplitudes, rescanning has beneficial effects and helped to improve dosimetry metrics for the investigated limited dp/p of 1.2% (realistic) and 2.4%. The median differences between ideal and respiratory-synchronised tracking show minimal discrepancies, 1% and 5% respectively for dose coverage (CTV V95) and homogeneity (D5-D95). Multiple-field planning improves D5-D95 up to 50% in the most extreme cases while it does not show a significant effect on V95. Significance. This work shows the potential of implementing tumour tracking in current proton therapy units and outlines design requirements for future developments. Energy regulation within momentum acceptance was investigated to tracking tumour motion with respiratory-synchronisation, achieving results in line with the performance of ideal tracking scenarios. ±5% Δp/p would allow to compensate for all range offsets in our NSCLC patient cohort, including breathing variability. However, the realistic momentum of 1.2% dp/p representative of existing medical units limitations, has been shown to preserve plan quality.

4D Monte Carlo dose reconstructions using surface motion measurements

PurposeTo investigate the feasibility of a 4D Monte Carlo based dose reconstruction method to study the dosimetric impact of respiratory motion using surface motion measurements for patients undergoing VMAT treatments for Non-Small Cell Lung Cancer. MethodsThe 4Ddefdosxyznrc/EGSnrc algorithm was used to reconstruct VMAT doses delivered to the patients using machine log files and respiratory traces measured with the RADPOS 4D dosimetry system. The RADPOS sensor was adhered to the patient’s abdomen prior to each treatment fraction and its position was used as a surrogate for tumour motion. Treatment log files were synchronized with the patient respiratory traces. Patient specific respiratory models were generated from deformable registration of the inhale and exhale 4DCT images and the respiratory traces.The reconstructed doses were compared to planned doses calculated with DOSXYZnrc/EGSnrc on the average-intensity and the exhale phase CT images. ResultsRespiratory motion measurements and log files were acquired for 2 patients over 5 treatment fractions each. The motion was predominantly along the anterior/posterior direction (A/P). The average respiratory amplitudes were 8.7 ± 2.7 mm and 10.0 ± 1.2 mm for Patient 1 and 2, respectively. Both patients displayed inter- and intra-fractional variations in the baseline position. Small inter-fractional differences were observed in the reconstructed doses for each patient. Differences between the reconstructed and planned doses were attributed to differences in organ volumes. ConclusionThe 4D reconstruction method was successfully implemented for the two patients studied. Small differences between the planned and reconstructed doses were observed due to the small tumour motion of these patients.

Investigation of uncertainty in internal target volume definition for lung stereotactic body radiotherapy

This study evaluated the validity of internal target volumes (ITVs) defined by three- (3DCT) and four-dimensional computed tomography (4DCT), and subsequently compared them with actual movements during treatment. Five patients with upper lobe lung tumors were treated with stereotactic body radiotherapy (SBRT) at 48 Gy in four fractions. Planning 3DCT images were acquired with peak-exhale and peak-inhale breath-holds, and 4DCT images were acquired in the cine mode under free breathing. Cine images were acquired using an electronic portal imaging device during irradiation. Tumor coverage was evaluated based on the manner in which the peak-to-peak breathing amplitude on the planning CT covered the range of tumor motion (± 3 SD) during irradiation in the left–right, anteroposterior, and cranio-caudal (CC) directions. The mean tumor coverage of the 4DCT-based ITV was better than that of the 3DCT-based ITV in the CC direction. The internal margin should be considered when setting the irradiation field for 4DCT. The proposed 4DCT-based ITV can be used as an efficient approach in free-breathing SBRT for upper-lobe tumors of the lung because its coverage is superior to that of 3DCT.

75 Heatmaps to Assess Tumour Motion Probability with Mrcine in Head and Neck Cancer

75 Heatmaps to Assess Tumour Motion Probability with Mrcine in Head and Neck Cancer

72 A Deep Learning Approach to Tumour Motion Forecasting for Surface Guided Radiation Therapy Based on Volumetric 4D-CT

72 A Deep Learning Approach to Tumour Motion Forecasting for Surface Guided Radiation Therapy Based on Volumetric 4D-CT

An analysis of the impact of different levels of inspiratory volume using active breathing control on the intrafraction motion and dose coverage of target volumes in patients undergoing thoracic radiotherapy

PurposeTo determine whether the level of inspiratory volume affect the extent to which active breathing control (ABC) reduces intrafraction motion and the dose coverage of target volumes in patients receiving external beam radiation therapy (EBRT) to the thoracic region Materials/methods20 patients undergoing thoracic radiotherapy enrolled in a prospective study using ABC for respiratory motion management and volumetric-modulated arc therapy (VMAT) as the treatment technique. They were randomized to one of two groups, the control group of 80% inspiratory volume and the other test group of 70%. At least one set of repeated CBCTs was done weekly. All images including CBCTs and Planning CTs were sent to MIM softwareTM for analysis of intrafraction motion using Dice Similarity Coefficient (DSC). The target dose conformality was assessed using conformation number (CN). Intention-to-treat analysis was employed for statistical purpose. ResultsThe DSC for the 70% and 80% inspiratory volume group were 0.93 and 0.92, respectively. For the 70% group, there was a significant negative correlation (p < 0.05) between DSC and time between two CBCTs, but not for the 80% group. The average percentage change in CN for the 70% and 80% group was 10.91% and 8.14%, respectively, and their difference was significant (p < 0.05). Furthermore, the actual change in volume had a significant positive correlation (p < 0.05) with the percentage change in CN for the 70% inspiratory volume group but not the 80% group. ConclusionMore evidence suggests that the target volumes from the 80% inspiratory volume group have less intrafraction motion compared to the 70% group. The findings from the percentage change in CN suggest that there could potentially be less tumor motion for higher levels of inspiratory volume and this could possibly contribute to why intrafraction motion is less for the 80% inspiratory volume group.

Diaphragm displacement estimation based on subspace mapping from markerless abdominal surface measurement

Respiratory induced organ and tumor motion has great influence in radiation therapy. In order to estimate the inner structures’ movement by external observation, a novel two-step subspace mapping (TSSM) algorithm is proposed to estimate the diaphragm displacement from markerless abdominal surface measurement. We first incorporate 3D image segmentation to measure the displacement of the inner diaphragm and the external abdominal surface on 4D CT images. TSSM constructs eigenspaces by PCA and projects the two organs’ displacement data into their corresponding subspaces. Then, TSSM makes a mapping between the subspaces by a Ridge regression optimization to effectively characterize their correlation. Based on the trained correlation model, the diaphragm displacement can be estimated by the abdominal surface without markers. TSSM is further extended to kernel TSSM (kTSSM) to investigate the non-linear correlation. Experiments show the proposed method can accurately estimate the displacement of the internal diaphragm by the external abdominal surface.

Comparison of dynamic tumor tracking error measurement methods for robotic radiosurgery.

Dynamic tumor motion tracking is used in robotic radiosurgery for targets subject to respiratory motion, such as lung and liver cancers. Different methods of measuring tracking error have been reported, but the differences among these methods have not been studied, and the optimal method is unknown. The purpose of this study was to assess and compare tracking errors encountered with individual patients using different evaluation methods for method optimization. We compared the beam's eye view (BEV), machine learning (ML), log (addition error: AE), and log (root sum square: RSS) methods. Log (AE) and log (RSS) were calculated from log files. These tracking errors were compared, and the optimal evaluation method was ascertained. A t-test was performed to evaluate statistically significant differences. Here, the significance level was set at 5%. The mean values of BEV, log (AE), log (RSS), and ML were 2.87, 3.91, 2.91, and 3.74mm, respectively. The log (AE) and ML were higher than BEV (p<0.001), and log (RSS) was equivalent to the BEV, suggesting that the log (RSS) calculated with the log file method can substitute for the BEV calculated with the BEV method. As RSS error calculation is simpler than BEV calculation, using it may improve clinical practice throughput. This study clarified differences among three tracking error evaluation methods for dynamic tumor tracking radiotherapy using a robotic radiosurgery system. The log (RSS) calculated by the log file method was found to be the best alternative to BEV method, as it can calculate tracking errors more easily than the BEV method.

Evaluation of tumour motion and internal/External Correlation in lung SABR.

This study aims to analyse lung tumour motion and to investigate the correlation between the internal tumour motion acquired from four-dimensional computed tomography (4DCT) and the motion of an external surrogate. A dataset of 363 4DCT images was analysed. Tumours were classified based on their anatomical lobes. The recorded gross tumour volume (GTV) information included the centroid GTV motion in the superior-inferior (SI), anteroposterior (AP) and left-right (LR) directions, and in three-dimensions (3D). For the internal/external correlation, the RPM surrogate breathing signals of 260 patients were analysed via an in-house script. The external motion was correlated with the 3D centroid motion, and the maximum tumour motion via Spearman's correlation. The effect of tumour volume on the amount of motion was evaluated. The greatest 3D tumour amplitude was found for tumours located in the lower part of the lung, with a maximum of 26.7 mm. The Spearman's correlation of the internal 3D motion was weak in the upper (r = 0.21) and moderate in the middle (r = 0.51) and the lower (r = 0.52) lobes. There was no obvious difference in the correlation coefficients between the maximum tumour displacement and the centroid motion. No correlation was found between the tumour volume and the magnitude of motion. Our results suggest that tumour location can be a good predictor of its motion. However, tumour size is a poor predictor of the motion. This knowledge of the distribution of tumour motion throughout the thorax will be valuable for research areas where motion management is still in its infancy.